When I try to play minecraft, it says "not supported on this architecture"

Apple how two Architectures, Intel and PPC.

PPC is (PowerPC) and was used in the PowerBook range of laptops.

Intel processor are used in the MacBook range of laptops.

When your getting an error like this, it means the application has been designed for the Intel architecture and that means it will not run on your PowerBook with the PPC architecture

Hope this helps

Please answer this question ASAP.

Please understand that you are talking to other end-users like you who volunteer to help here, not Apple.

Is there any way I can play minecraft on my PowerPC?

See this thread and this thread.

You should consult the minecraft site/forums/wiki, as there are a number of recommended configurations, both miniimum and better performance. Note that "cheap" and "no lag" may be somewhat contradictory, however, anything is possible in the used market.

Also, this discussion area is probably not a good one to ask in, if you intend on staying with a mac system to play the game. You might ask in the macbook discussion here or macbook air discussion here or macbook pro discussion here. (Assuming you want to stay with a notebook computer).

where I can buy...

Hard to say not knowing where you are. If you tell us an approximate geographic location, someone may know a resource in your area.

Also, do you have the system requirments for the game in question? I'm not familiar with it.